1. Field of the Invention
The present invention relates to a method of measuring a very small quantity of impurities in a gas.
2. Description of the Prior Art
High-purity halogen gases, hydrogen halide gases and inorganic hydrides have been used as dry etching gases in the production of semiconductors. Various kinds of gases such as SiH.sub.4, Si.sub.2 H.sub.6 and SiH.sub.2 Cl.sub.2, have been used in the formation of thin films of Si, SiO.sub.2, Si.sub.3 N.sub.4 and the like but even though a remarkably very small quantity (for example a ppb-level) of impurities may be contained in these gases, a bad influence upon a manufacturing process of LSI and the like by these impurities is increased as a circuit pattern is reduced to line details and also a gas piping system itself can be seriously influenced. So, it has been desired in the industry to accurately measure any concentration of the impurities at a ppb-level and to control them.
A method of detecting a dew point of a gas to calculate a concentration of impurities on the basis of the dew point has been known as the conventional method of measuring a concentration of very small quantity of impurities, for example water, contained in a gas. For example nitrogen gas, which is one of the inert gases, can be measured until a dew point of -90.degree. C. (about 95 ppb in a concentration of impurities).
However, the halogen gases and inorganic hydrides themselves have high dew points, so that a lower concentration of water has been unable to be detect by the above described method. So, it has been thought to use an atmospheric ionization mass spectrometer (APIMS). Although a remarkably very small quantity of impurities can be measured by APIMS, it is not only expensive but also large-sized, so that a disadvantage has occuured in that an installing site is limited.
In addition, an ionization process by a corona discharge under an atmospheric pressure is comprised, so that a disadvantage has occurred in that a direct measurement is impossible and thus an accurate measurement cannot be achieved due to a corrosion in case of the halogen gases and for example an accumulation of Si and the like in case of reactive gases such as SiH.sub.4.